1. Field of the Invention
The present invention relates to a piezoelectric vibrator, and more specifically to a surface mount piezoelectric vibrator in which a piezoelectric vibrating piece is sealed in a cavity defined between two bonded substrates.
2. Description of the Related Art
In recent years, mobile telephones and portable information terminal devices employ a piezoelectric vibrator using quartz crystal as a time source, a timing source of control signals, and a reference signal source. As this type of piezoelectric vibrator, various ones are known. As one of them, there is a piezoelectric vibrator of a surface mount device (SMD).
As a typical surface mount piezoelectric vibrator, a three layer structure type is generally known in which a piezoelectric substrate formed with a piezoelectric vibrating piece is sandwiched between a base substrate and a lid substrate from above and below. In this case, the piezoelectric vibrator is accommodated in a cavity (airtight chamber) defined between the base substrate and the lid substrate. In addition, in recent years, two layer structure types are also developed, different from the three layer structure type.
The two layer structure type piezoelectric vibrator is formed to have a two layer structure by directly bonding a base substrate to a lid substrate, in which a piezoelectric vibrating piece is accommodated in a cavity defined between these two substrates. The two layer structure type piezoelectric vibrator is excellent for preferable use in that its thickness can be reduced as compared with three layer structure ones. As one of those two layer structure type piezoelectric vibrators, such a piezoelectric vibrator is known that a conductive member that is formed so as to penetrate through a base substrate is used to bring a piezoelectric vibrating piece and an external electrode formed on a base substrate into conduction (for example, see JP-A-2001-267190 (Patent Document 1) and JP-A-2007-328941 (Patent Document 2)).
In the piezoelectric vibrators described in Patent Document 1 and Patent Document 2, a through hole is formed in an insulating base substrate formed of ceramics or glass. Then, the conductive member is buried in the through hole such that the through hole is blocked. The conductive member is electrically connected to the external electrode formed on the undersurface of the base substrate as well as electrically connected to the piezoelectric vibrating piece accommodated in a cavity.
In the piezoelectric vibrator, the conductive member serves for two main roles: a) blocking the through hole to maintain hermeticity in the cavity, and b) bringing the piezoelectric vibrating piece and the external electrode into conduction. When the through hole is insufficiently contacted with the conductive member, hermeticity in the cavity is sometimes impaired. In addition, when electrical connection of the conductive member to the piezoelectric vibrating piece or to the external electrode is insufficient, this poor electrical connection causes malfunction of the piezoelectric vibrating piece. Therefore, in order to eliminate such failure, it is important to form the conductive member in such a way that the conductive member fully blocks the through hole as firmly contacted with the inner surface of the through hole as well as no dent is made on the surface.
When a conductive paste is used as a conductive member, it is necessary that the conductive paste is buried in the through hole, fired, and cured. However, when the conductive paste is fired, organic substances contained in the conductive paste are evaporated and lost, and thus the volume after fired is generally reduced as compared with the volume before fired (for example, when Ag paste is used as a conductive paste, about 20% of the volume is reduced). On this account, even though a conductive paste is used to form a conductive member, a dent occurs in the surface, or in the worst case, a through hole is formed at the center. Consequently, a problem arises that hermeticity in the cavity is impaired, or continuity between the piezoelectric vibrating piece and the external electrode is impaired.
In order to solve this problem, such a method is proposed in which a metal pin is disposed in a through hole, and a paste material such as a glass frit is filled in a clearance between the through hole and the pin and fired to form a through electrode. The through electrode is formed in this manner, whereby the volume is reduced only in the portion of the paste material. Therefore, the time required for the polishing process after that can be shortened, and the through electrode can be efficiently formed.